1. Field of the Invention
This invention relates to spread spectrum communication systems and more particularly to a receiver for decoding several signals having different spreading functions concurrently using a single convolver.
2. Description of the Prior Art
M-ary Orthogonal Keying (MOK) data modulation is a technique used in spread spectrum communications where one basic spreading function, such as a pseudo noise (PN) sequence is phase modulated on a carrier. The spreading function is modified by certain other functions, such as Walsh functions to cause a modulation orthogonal to the basic function and every other modulating function used. The resultant response of a signal in a channel filtered in a matched filter to an orthogonally modulated signal is zero, while the response in a matched filter to nonorthogonal signals is maximum. The modulation of the spreading function with orthogonal functions is used extensively in secure communications, both for continuous communications and short message formats. Spreading the small amount of information pseudo randomly over a wide bandwidth allows a low probability of intercept (LPI) since the signal power density can be below the noise power density. Only by knowing the spreading function can the signal energy be integrated above the noise.
Presently, orthogonal keyed data is recovered by implementing parallel decoding devices, such as correlators or convolvers and then comparing the filtered amplitudes as in a Mark and Space receiver. Using higher M-ary than binary orthogonal keyed (BOK) data modulations at low bandwidths, where processing at base band is practical, parallel processing for decoding the orthogonal modulations using partial sums from a single decoder of the basic PM spreading function is sometimes implemented.
For medium bandwidth modulation, where the spreading function is changing after short intervals, convolvers are generally used. One example of a surface acoustic wave convolver is described in U.S. Pat. No. 4,428,062 which issued on Jan. 24, 1984 to P. A. Michaels and assigned to The Bendix Corporation. In '062 two transducers are shown spaced apart for launching surface acoustic waves along a path between the two transducers. The interaction of the surface acoustic waves causes a voltage at each point which is collected by a metal conductor overlying the interaction region. A convolver such as described in '062 is capable of much wider bandwidths than can be obtained in base band processing. At present the excess bandwidth over that required for modulation decoding is not used.
It is therefore desirable to utilize the available bandwidth of a convolver by frequency multiplexing several signals within this bandwidth.
It is further desirable to provide convolution of an input signal on four different i.f. carriers with four orthogonally modulated reference signals on four i.f. carriers in a broadband convolver.
It is further desirable to utilize a combiner for combining several input signals at different frequencies to one input of a broadband convolver while coupling the other input of the convolver with a plurality of reference signals at the same frequencies, but with respective time reversed modulations.
It is further desirable to provide a spread spectrum communication system having a broadband convolver which is simultaneously used for convolving a plurality of input signals with a plurality of reference signals.
It is further desirable to filter the output of a broadband convolver simultaneously convolving a plurality of signals to recover the convolution of the input signal with selected reference signals.
It is further desirable to provide a spread spectrum communication system having an input signal modulated by at least one of a plurality of predetermined orthogonal modulation functions by simultaneously convolving the input signal with a plurality of reference signals, each having a respective modulation and for recovering the convoluted data by filtering and detecting the output of the convolver for each respective modulation function that correspond to different carrier frequencies.